SCR　is　one　of　the　best　after　treatment　technology　for　de-NOx　in　automotive　diesel　engines.　However,　due　to　the　sea　water　scrubbing　desulfurization　in　marine　diesel　engines,　the　exhaust　gas　leads　to　a　significant　decrease　in　exhaust　gas　temperature,　so　SCR　technology　for　vehicles　cannot　be　directly　transferred　to　marine　diesel　engines.　This　work　conducted　a　bench　test　to　test　the　exhaust　emissions　of　a　diesel　engine　by　adding　ozone　to　the　exhaust　gas　under　conditions　of　both　low　load　and　idling.　The　results　showed　that　when　the　exhaust　gas　temperature　was　above180 ,　NOx　increased　with　a　maximum　increase　of　93.14%　due　to　the　ozone　decomposition.　When　the　gas　temperature　was　below　100 ,　ozone　oxidized　part　of　NO　to　NO2.　The　O3/NO　molar　ratio　had　a　greater　effect　on　the　NOx　emissions　of　diesel　engines.　As　the　O3/NO　molar　ratio　was　close　to　1,　the　NO　oxidation　effect　was　the　best　and　the　de-NOx　efficiency　reached　79.31%.　Under　low　temperature　operating　conditions,　O3　is　effective　in　reducing　NOx　emissions　from　the　exhaust　of　marine　diesel　engines.　©　2023,　The　Author(s),　under　exclusive　license　to　Springer　Nature　Singapore　Pte　Ltd.